Who is it?

I've photographed what for me is a 'new' bird, but I'm not sure what it is. I think it might be a Garden Warbler (Sylvia borin), or a Chiffchaff (Phylloscopus collybita). Or A N Other?! Any thoughts gratefully received. It was pootling around in the middle of a bramble hedge - which both of these species do! Watched it for some minutes towards the end of my walk this afternoon (coatless again!) on this lovely mild, hazily sunny day. (The beaches were enveloped in sea mist and were probably quite cool, but I was walking about 250-300 feet above sea-level.)

Good news: I selected One for Arthur to win the Grand National -- and it did!
Bad news: I didn't place a bet. (14:1, as well!)

I was going to have my annual flutter, but then I decided it was such a beautiful day, it would be a shame to drive into town, struggle to find somewhere to park, etc. and I'd rather enjoy the sunshine in the  garden and countryside. I still got excited when it won, mind you! And that's the main fun. (More importantly still, all horses survived - and their jockeys!)

EDIT: to add that I'm increasingly swinging towards 'chiffchaff' as my identification of this bird BUT if this is a chiffchaff, then what is the bird I blipped last September that I thought was a chiffchaff? They do differ!

The plot thickens!!!